This invention relates generally to secure communication systems and, more particularly, to cable television systems wherein designated subscribers are enabled to receive particular program material.
In any such subscriber television system, means are required for scrambling the audio and video information, and means are also required for transmitting program and subscriber information to designate subscribers permitted to view particular programs. Although the prior art discloses a wide variety of methods, commercially accepted systems typically perform these functions by independent analog scrambling of the audio and video information, and by multiplexing digital customer and program data with either the audio or video signals. Popular techniques include, for example, "sine-wave scrambling" of the video to suppress horizontal synchronization and modulating the audio information on a supersonic subcarrier. More advanced commercial systems employ a pseudo random code for scrambling the video information, for example, by polarity inversion of the video signal on a frame-by-frame or line-by-line basis. These techniques have required increased complexity and cost, but the additional security is needed to frustrate pirates who have gained considerable skill and experience in circumventing security measures.